Judging the probability of uncertain events is essential for effective decision making. Yet people make many systematic errors in their intuitive judgments of probability. These errors impede important decisions that people make on a daily basis, such as medical diagnoses, political and legal policy-making, and planning for the future. The specific aims of the proposed research are to evaluate and explore the implications of a conceptual framework developed by Gavanski and Hui (in press) for understanding intuitive judgments of probability. Gavanski and Hui have argued that both accurate and inaccurate probability judgments can be understood according to what sets of information people access from memory for the judgments. The proposed research will (a) test some basic assumptions underlying the framework, (b) apply the framework to understanding the processes by which cognitive organization (i.e., the categorization of information in memory) influences people's ability to judge various probabilities, and (c) evaluate novel predictions of the framework for conditions under which people will make either accurate or inaccurate judgments of probability. The long-term objective of the proposed research is to provide an understanding of how people judge probability and when and why they make judgment errors. In addition, the research will lead to the identification of tasks particularly likely to evoke judgment errors and to the development of techniques for improving people's judgments and decisions.